The present invention relates to a steam cooking apparatus and in particular to a connectionless steam cooker with an automatic cold air and steam outlet and close pressure control to enhance the efficiency of the cooker.
A wide variety of steam cookers are known. Market Forge Industries, Inc., the present assignee, manufactures a wide variety of steam cookers and food warmers that have found wide acceptance in the food service industry as a way to rapidly cook food, including frozen food, and/or maintain it at a serving temperature. While pressure cookers have long been used in homes and restaurants, the risks and extra costs of containing steam under high pressure have led to the growth of slow steam cookers that use steam to deliver heat to the food, but at a pressure that is typically just above atmosphere (1 to 2 inches of water). Most conventional slow (or xe2x80x9cpressurelessxe2x80x9d) steam cookers have steam generators, typically boilers external to the cooking compartment and using electrical resistance or gas heaters that produce steam from a water supply.
U.S. Pat. Nos. 5,549,038 and 5,631,033 to Kolvites and assigned to Market Forge Industries describe one implementation of a commercial slow steam cooker. The Kolvites cooker has permanent connections to a water supply and a drain. It can continuously replenish the supply of cold water. Periodically, on demand, it can draw water from the supply, heat the water to produce steam, circulate the steam in an oven chamber around the food to cook it and/or keep it warm until served, and then direct the steam and condensed water to the drain.
While such units offer many advantages, such as rapid, efficient cooking of large volumes of food, including frozen food, their disadvantages include the need for water and drain hookups, a relatively large water usage, attendant high power requirements to heat the water, and burdensome maintenance requirements such as daily, monthly and annual cleanings to remove scaling (xe2x80x9cdelimingxe2x80x9d) on heating coils, tubes and other components produced by the boiling, as well as to remove residue from the cooking process itself. These units are typically large and comparatively complex in their construction. They also are constrained in that a flow of steam is condensed to hot water that must usually be cooled before it can be drained into public sewer systems.
Another type of steam cooker, commonly termed xe2x80x9cconnectionlessxe2x80x9d, avoids some of these constraints of connected cookers. Connectionless cookers, as the term suggests, do not have permanent connection to a water supply. Rather, water is added manually to the unit. It is evaporated, condensed, collected, and reused. This type of steamer can operate for comparatively long periods of time without adding additional water, and with significantly reduced water and power usage as compared to the connected steam cookers because cold water is not added on demand to form the steam, and then drained after use. An earlier steamer sold by Market Forge Industries, Inc. under the trade designation xe2x80x9cSTEAM ITxe2x80x9d is connectionless and holds a supply of water within its heating compartment. In general, connectionless steamers cook smaller quantities of food than connected ones, but are easier to maintain, more portable, and cost less to operate.
Steam cookers of both typesxe2x80x94connected and connectionlessxe2x80x94use a door to gain access to the cooking compartment to add and remove the food, typically food held in one or more pans that slide onto racks mounted on the side walls of the cooking compartment. The opening and closing of this door produce a loss of steam, fluctuations in steam pressure, and introduce cold air into the compartment. The efficiency and the quality of operation of the steam cooker is dependent upon the degree to which the temperature and pressure of the steam within the cavity can be maintained at or near a preselected optimal value, or within an optimal range of values.
It is also important in all steamers to have reliable and effective controls to prevent burnout of the heater, typically caused by a low water condition. In connectionless steamers, a critical low water situation typically develops as steam is lost from the cooking compartment, and more water is boiled to replace it. (There is usually loss at least when the door is opened, and via a bleed orifice used to introduce some flow in the steam to keep it xe2x80x9cactivexe2x80x9d.) Eventually the water supply is depleted, causing the electric resistance heating element to overheat. Known connectionless steamers have water level detection arrangements, but they can fail. One particular problem is that certain foods when steam cooked release materials to the steam which collect in the condensed water and create a layer of foam on the water. This foam can interfere with the operation of the water level detectors, causing the heating element to overheat to the degree that causes permanent damage. Another problem with known connectionless steamers is that scaling (mineral deposits) produced by evaporation of the water, as well as the accumulation of residue from the cooking process, can be carried by fluids and interfere with the operation movement of moving components used to control the cooking process.
Another problem with steamersxe2x80x94and particularly connectionless steamers that are inherently closed systemsxe2x80x94is that on start-up, or after the door is opened during cooking, cold air is trapped in the cooking compartment. The cold air takes heat energy from the cooking and makes the heat gradient and cooking rate within the over uneven. While mechanical steam vents, bleed orifices, check valves, and the like have been used, the rapid and controlled elimination of trapped cold air from the cooking compartment, without also losing any significant volume of steam, remains a problem.
The amount of heat produced by a steam cooker also must accommodate variations in the quantity of food, its temperature, and its surface area. A small amount of room temperature food will be cooked quickly with the steam generator powered. Continued heating will generate a dangerous overpressure and overcook the food. Various arrangements have therefore been employed to apply electrical power to a heater of a steam generator intermittently, as needed. The aforementioned Kolvites ""038 and ""033 patents, for example, use a pressure-sensitive switch connected in series in the power supply line. The switch responds to the steam pressure in a long outlet conduit from the oven. To control a possible dangerous outrush of steam when the oven door is opened, Kolvites provides a switch responsive to an opening of the door that opens a valve in a fresh water supply line. The resulting cold water flow quenches steam in the steam generator and also cools a mechanical steam trap to open the steam outlet line to atmosphere.
In connectionless steamers, Creamer et al, U.S. Pat. No. 5,869,812 disclosed a float switch that controls the application of power to a heater mounted under the floor of a heating chamber, adjacent a pool of water. The float switch operates by balancing atmospheric pressure on a supply of water in a reservoir external to the steam housing against the steam pressure in the cooker carried by a conduit from an upper portion of the chamber to the float switch. Depending on how this balance is struck, power to the heater is on or off. Creamer et al. slope the floor of the heating chamber to one corner to facilitate drainage of the pool of water held there. Outlet steam is condensed and cooled in the reservoir, and then gravity recirculated by a conduit back to the water pool within the heating chamber.
In U.S. Pat. Nos. 6,175,100 and 6,107,605, Creamer et al. propose a solution to the problem of cold air trapped in the connectionless cooler of the ""812 patent. They place a small hole (bleed orifice) in the steam outlet conduit leading to the float valve. The hole is continuously open to atmosphere. Cold air may escape, as may steam.
It is a principal object of the present invention to provide a connectionless steam cooker that eliminates trapped cold air rapidly from the steamer while at the same time conserving water and steam and closely maintaining a preselected temperature and pressure range within the steamer.
A further object is to provide these advantages with a simple, reliable construction.
Another object is to provide a simple and reliable low-water detection system that is not sensitive to scaling, food debris or food foam.
Still another object is to provide these advantages together with ease of cleaning and maintenance, low water usage, low power consumption, and operator safety.
A connectionless steam cooker has a housing with insulated bottom, top and side inner and outer walls that define a cooking compartment or oven within the inner walls. The floor of the oven and adjacent portions of the side walls hold a supply of water. A heater, in one form a plate-like assembly of cast metal with electrical heating elements embedded therein, is secured at the floor of the housing in a water well. In one form, this securing is replaceable, using a set of threaded studs that engage holes in a peripheral flange, with an edge seal gasket to hold water in the cooking compartment. The heater is in direct contact with the water for efficient heat transfer. Operation of the heater generates steam that circulates through the oven.
A steam outlet is formed in a wall of the housing (e.g., a back wall of a cooking compartment) and in a lower half of the cooking compartment. In one form, it is located preferably immediately above the high water level in the cooking compartment so that steam generation drives cold air down to this outlet. An electrically-operated solenoid valve is connected substantially directly to the steam outlet via a branched outlet conduit. One branch leads to the solenoid valve and the other branch leads to a pressure regulation/power control system.
The solenoid valve is normally open to atmosphere. A thermostat that senses the temperature of the steam in the heating chamber is located adjacent the steam outlet, also just above the high water line. The thermostat is closely controlled to produce an output control signal when the sensed temperature is at or above a preselected set point, typically just under the boiling point of the water. This output signal closes the solenoid valve to block any significant loss of steam to the atmosphere while directing the outlet vapor via conduits to a set of pressure-sensitive microswitches. The thermostat and solenoid valve so located and so connected constitute an automatic electric steam trap that also provides a ready path to eliminate cold air from the steamer. It is also rapidly and accurately responsive to the temperature of the steam in the heating compartment, e.g., that it is 205 to 210xc2x0 F. in the cooking compartment.
The second branch conduit from the steam outlet can be a single conduit that itself branches into two conduits. One branch directs the outlet vapor to a first or operating microswitch that cycles between xe2x80x9cONxe2x80x9d and xe2x80x9cOFFxe2x80x9d states as sensed pressure moves between lower and upper preselected set pressure values, typical ones being 1 and 3 inches of water column pressure (xe2x80x9cW.C.xe2x80x9d), the normal operating range during cooking. When the steamer is started, power is supplied through a preheat stage until it generates steam, and the steam fills the cooking compartment at the preselected operating pressure. As the sensed pressure rises to the upper limit, the first switch interrupts the supply of electrical power to the heater. As the steam then cools, the pressure falls until the falling pressure trips the lower limit of the first switch to supply power again.
The other branch conduit directs outlet steam vapor from the steam outlet to a second pressure-sensitive microswitch that is set to open the solenoid valve on rise at a pressure valve above an upper limit. For an upper limit of 3 inches W.C. at the first operating switch, a typical value of the set limit point of the second switch is 9 inches W.C. This second conduit branch and second switch is a safety feature to release steam if the pressure in the cavity exceeds a safe pressure relief. If this arrangement should fail, a check valve separate from the solenoid valve and in the second branch line from the steam outlet opens to release steam from the steam outlet to atmosphere.
The plate-like heating element is mounted within the cooking compartment at its floor or so as to be immersed in the cooking water when the steamer is operating. This provides an efficient heat transfer to the water. The heater is also tilted with respect to the horizontal (defined by the water level). This tilt is preferably along a diagonal of a rectangular plate heater so that one comer is elevated slightly with respect to the other comer. Heat sensors, e.g., snap disc type sensors, are mounted on these raised and lowered portions of the heater. As water is lost, the raised (xe2x80x9cprimaryxe2x80x9d) sensor detects a temperature rise (e.g., to 310xc2x0 F.) which can equal a low water condition. The switch automatically resets. If the lower sensor detects a temperature rise (e.g., to 378xc2x0 F.), then it also signals a low water condition, turning off the power, and must be manually reset.
The cooking cavity has a wire rack between a lowermost pan in the cooking compartment and the maximum water level. The rack is sufficiently rigid to hold food to be warmed, sufficiently porous to promote the rise of steam into the cavity, and formed of sufficiently closely spaced wires to block an operator""s hand from touching the hot water.
The power controls include a two-level power supply selector switch. In one position, the system supplies fill power to the heating element, and in the second position supplies a reduced power, e.g., 6 KW reduced to 4 KW for a three-pan sized cooking compartment.
These and other features and objects of the present invention will be better understood from the following detailed description of the invention which should be read in light of the accompanying drawings.